We propose to study the action of the Ca-ionophores X537A and A23187 on the sarcoplasmic reticulum (SR). Effects on Ca-binding and Ca-uptake will be characterized in terms of a possible reaction mechanism that involves the formation of a calcium-membrane complex and the movement of this complex into the SR. The actions of a cyclic AMP-dependent protein kinase to stimulate Ca-transport will be analyzed in the context of these studies, and the nature and role of phospholamban, the phosphate acceptor of the cardiac SR, will be studied.